Thermal fatigue

Chapter 7 will help the analyst understand how thermal residual stresses are formed.
The basic principle to recall is that the metal that cools last will have tensile residual stresses; also, for thermal residual stress to occur, it is necessary to have both heat and restraint. If, for example, a sharp edge of a part is repetitively heated and cooled while the restraining bulk of the part remains relatively cool, the sharp edge will expand when heated and contract when cooled. That is, the sharp edge will develop compressive forces and yield compressively when hot because of the lower strength and lower modulus of elasticity at the elevated temperature. When the edge is cooled, tensile residual stresses will form; if this action is repeated many times, thermal fatigue cracks will develop and will tend to grow each time the metal is cooled from the elevated temperature, for it is at that time that the tensile residual stress is again applied. When the metal is hot, the cracks will tend to close, although this will probably be resisted by an accumulation of oxide scale or other products of combustion from the high-temperature atmosphere. In fact, these products in the crack can act as a wedge and increase the compressive yielding when hot, so that the tensile residual stress that forms on cooling is increased.
Since both heat and restraint are necessary for thermal residual stresses to occur and repetitive thermal cycling is necessary for thermal fatigue, the methods to prevent such fracture should become obvious. If restraint is internal within the part, as described above with the sharp edge, it may be possible to reduce the restraint by permitting the part to expand more uniformly when it is heated, instead of having just the sharp edge expand. If this is not practical, it may be possible to blunt the sharp edge by changing the shape, so that this action is not concentrated on a thin section. Or it may be possible to reduce the thermal gradient within the part by permitting more of the metal to be heated, or to design curves into the part so that expansion and contraction forces simply change the shape of the part instead of developing potentially destructive tensile residual stresses. This is the reason that expansion loops are designed into high-temperature piping, such as steam lines. It is also the reason that expansion joints are necessary in bridge and road design.
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